Electronic devices operating in the terahertz (THz) frequency range will enable functionality in numerous applications, including radio astronomy, earth remote sensing, radars and vehicle radars, scientific investigations, non-destructive testing of materials and electronic devices, chemical analysis, explosive detection, moisture content, coating thickness control, imaging, wireless covert communications, and/or the like. In spite of great demand for efficient THz devices, compact and efficient transistor THz devices are not yet available.
Achieving electronic device operation in the THz range is a complex multifaceted problem involving the control of electron velocity, electric field and potential profiles, access resistances, parasitic parameters, and electromagnetic coupling. To date, electronic devices such as photomixers and frequency multipliers can only deliver radio frequency (RF) powers in the micro-watt range when operating at frequencies in the THz range. Electronic THz lasers can emit high powers, e.g., up to one Watt, however, these devices are bulky, require high pumping powers and cannot be fabricated using integrated technology.
One of the most important criteria for an efficient THz emission is the peak electron velocity. Of the commonly used III-V materials (materials comprising elements from group III and group V), indium gallium arsenic (InGaAs), indium nitride (InN), and gallium nitride (GaN) have the highest peak electron velocity-electric field dependencies. In very short-gate devices, the average electron velocities under the gate might be considerably higher than the steady state values, due to so-called overshoot effects. The overshoot electron velocities for InN and GaN are expected to be close to the steady state electron velocity of InGaAs. However, in InN material, the electrons reach peak velocities at lower electric fields, thereby allowing for highest average velocities at a longer gate length than GaN-based devices (e.g., approximately three times longer).
High electron mobility transistors (HEMTs) with cutoff frequencies in the THz or sub-THz range have been demonstrated. Advances in silicon (Si) technology, have enabled the fabrication of metal oxide semiconductor (MOS) devices with cutoff frequencies in the lower THz range. However, to date, high power THz operation has not been achieved. Key obstacles to providing high power, high frequency operation include relatively low current density in Si, GaAs and InP based devices, rapid degradation of cutoff frequencies with increasing drain bias (due to short-channel effects), and low operating voltages in devices with submicron inter-electrode spacing.
GaN-based heterostructure field-effect transistors (HFETs) can comprise high electron densities and high peak electron velocity and mobilities in the two-dimensional electron gas (2DEG) channel. These properties indicate a possibility of operation at cut-off frequencies in the THz range with high output powers using nanoscale (e.g., approximately 30 nanometer) gate technology. As a result, GaN-based HFETs are excellent candidates for high-power solid-state THz sources. To date, such operation has not been achieved due to an effective gate length increase when the HFET is operated at high drain bias and access resistances, which causes significant degradation in the maximum and cutoff frequencies.
In particular, the effective gate length of a GaN-based HFET significantly exceeds the physical gate length when the HFET is operated at a high drain bias. The difference is due to an expansion of the 2DEG space charge region into the gate-to-drain spacing as the drain bias is increased. For a GaN-based HFET with an actual gate length of 0.15 μm, the effective gate length reaches 0.25 μm at a drain bias of 14 Volts (V) and approximately 0.5 μm at a 32 V drain bias. One approach incorporates an additional field-controlling electrode (FCE) to a GaN-based HFET, which is located in the near vicinity of the drain-side gate edge to control the extension of the space charge region and thus the cut-off frequencies at high drain bias.
However, limitations relating to the access resistance also restrict the microwave performance of GaN-based HFETs. In GaN devices, the lowest achievable contact resistance values are around an order of magnitude higher than contact resistance values achievable in GaAs devices. Contact annealing in GaN devices also requires much higher temperatures than the temperatures required for GaAs devices, which leads to rough contact edges and requires a larger gate to ohmic spacing to avoid premature breakdown and inter-electrode shortening. The source access resistance, which includes the contact resistance and the source-gate opening resistance, significantly reduces the external transconductance of sub-μm gate GaN devices and leads to lower drain saturation currents. In addition, the total source and drain access resistances increase the knee voltage for a GaN device, thus requiring higher drain voltage to operate the device and to achieve high RF powers. Another problem associated with the access region is depletion of the 2DEG due to surface potential modulation. As a result, the carrier concentration in the channel outside the gate becomes lower than that under the gate at high positive input signals. This leads to lower power gain and to an increase in the effective gate length at large input signals.
One approach seeks to significantly decrease the contact resistance to enable very tight source-gate-drain spacing in GaN-based HFETs by using capacitively coupled contacts. In this approach, a contact was formed directly on the source, and extended beyond the source into the source-gate region. Similarly, a contact was formed directly on the drain, and extended beyond the drain into the drain-gate region. The capacitively coupled contacts are included in microwave HFETs and provide low contact resistance during operation at microwave frequencies and provide independent control of induced carrier concentration in the source-gate and gate-drain openings.